I used to be like that, but one day I had enough and tried out CBT, cognitive behavioral therapy.
It has a fancy name, but it basically just involves 'thinking' counter-thoughts to negative thoughts and feelings.
Most people with SAD
know that their thoughts and feelings are often (almost always I'd venture) logically flawed and/or incorrect, so getting into the habit of simply challenging them usually helps greatly.
The idea is simple biology; the human brain is fairly malleable by evolutionary design. It is the ultimate adaptation tool that we have. While this may be made up (lol), from what I've read if you interrupt a particular pattern of firing neurons i.e. a thought and think more positive/realistic thoughts, your brain really has no choice but to slowly adapt.
The only downside is it takes some time, perhaps months, but the effects last much longer; after a few months, even a week, your brain will have likely changed its state and you will tend to feel better.
You have to understand that your feelings/emotions are really just simple raw data that your brain has processed for you in an easy and, the kicker, quick to understand format.
In short, your emotions don't necessarily reflect reality, and realistically only provide information suitable for your survival/pleasure within a local scope.
That being said, emotions are still very powerful. If you are struggling, start CBT, talk to a doctor, get on medications, meditate, anything to begin changing your brain. I know with SAD/depression/etc. it can be difficult reaching out for help but you have to find it in yourself to use some of your executive control and override your emotions and thoughts and take a first step.
The main point I want to get across is the state your brain is in is just that, a state. It is a remarkable organ, its malleability the likely contributing factor in our species success. So begin using its properties to your advantage.
Another point is that thoughts and emotions, excluding deliberate conscious thought (though that may be up for debate) are often autonomous, arising out of apparently nothing and being fed into your consciousness by apparently a 'daemon'. Treat them as such, simple suggestions spun together from the repository of all your past experiences, perhaps instinct, and perhaps a little randomness. You'll learn to appreciate and distinguish between feelings and thoughts you can trust and the illogical, vestigial ones that serve absolutely no good.
You'll probably also learn that most of these negative thoughts and feelings arise from half truths; they will be true, excruciatingly so, however their conclusions are often based upon false premises or faulty logic.
Any way, I started to turn around with CBT and proper medication. Who would've thought that simply thinking/focusing properly could help so much?
You can start by challenging/interrupting your negative thoughts/feelings. Question what you are thinking/feeling, why, and see if there is some sort of positive, more realistic thing you can grasp on too instead. Trust me, you'll find something that is at the very minimum more grounded in the real world.
In the beginning you'll find you'll be doing this all day, and it can be frustrating and tiring, but keep at it. You'll find that the positive thoughts will become more automatic, perhaps even unwanted

, in a surprisingly short amount of time. I believe it was a week, even a few days before I started noticing a subtle change in my thought process. With a change in thought comes new patterns of neural firing, which led to rewiring, etc.
Search for "google tech talks" on youtube to listen to researchers talk about this process, it is really interesting.
Get on meds if you need em', and start learning about CBT and other techniques; your brain will be rewired in no time.